1. Field of the Invention
The present invention relates generally to semiconductor fabrication methods and structures and, more particularly, to formation of a small contact hole with a high aspect ratio.
2. Description of Related Art
Current memory products may have a multi-layered, i.e., three-dimensional, structure that requires interconnection between elements on layers that are separated from each other by insulating material. For example, providing a connection between a first element disposed on a first layer and a second element disposed on a second layer may be achieved by disposing a contact hole between the first and second elements, the term “contact hole” being generic for a semiconductor etch of interest. Once the contact hole is in place, filling the contact hole with conductive material (e.g., metal) may provide the desired electrical interconnection between the first and second elements. When a length, L, separates the first and second elements, and when a contact hole between them has a width, w, then an aspect ratio of the contact hole may be defined as a quotient, L/w.
As memory products become smaller, integrated circuit elements necessarily become more closely spaced, thus requiring formation of contact holes with reduced cross-sectional areas and/or increased relative lengths. Each of these possibilities corresponds to an increase in the aspect ratio, L/w. Other things being equal, the increase in aspect ratio necessarily results in a decrease in conductance of a connection, with conductance being given by σA/L in which σ is the conductivity of the conductive material used to fill the contact hole and the cross-sectional area, A, of the contact hole generally decreasing as the square of the width, w.
High-aspect-ratio contact holes create fabrication difficulties in at least two ways. First, formation of the holes can be difficult because of their smaller geometry. For instance, the sidewall shape may suffer from a defect during etching such as via creation of a sub-optimal (e.g., reentrant) profile on the interior wall of the contact hole. Second, filling the smaller and/or longer holes with conductive material (e.g., metal) can become increasingly difficult with miniaturization because of known limiting factors such as stoichiometric/geometrical/dimensional considerations involved in uniformly and completely filling (e.g., by vapor deposition) all operational regions, including those deepest, of the contact hole with a suitably conductive material. As an example, a sub-optimal profile on the interior wall of a contact hole can cause partial failure in the filling of the contact hole in the subsequent processing of forming an electrically conductive layer. An improperly or incompletely filled contact hole may result in reduced or improper conductance of a connection, which in extreme cases may create an unacceptable condition known as a blind contact.
A need thus exists in the prior art for a method of reliably forming a relatively small contact hole which has a relatively high aspect ratio yet is capable of performing according to ever increasing stringent operational requirements. Along that vein, methods for filling small contact holes with conductive material should be implemented in manners no as to avoid blind contact issues.